Most types of integrated circuits (“chips”) require packaging to encapsulate the sensitive chips and provide mechanical protection during shipping, assembly and subsequent use. Optical chips, such as image sensors, are unique in that their packaging includes a transparent window to admit or release light. The window allows light to act upon the optical sensor array, which forms part of the chip. The transparent window, or lid, is commonly formed from glass. Optical devices of this type are difficult to manufacture because of stringent requirements for cleanliness. Any foreign material on the surface of the sensor array can cause image degradation which leads to rejection of the device and higher component cost due to reduced manufacturing yield.
Image sensor packages are typically formed by mounting a plurality of sensor chips in a rectangular array on a substrate tile (e.g., a ceramic tile). After wire bonding, a lattice of “dam” walls is formed using liquid epoxy between the adjacent chips so that each chip is surrounded on all four sides by a wall of epoxy. A glass sheet is then adhered to the tops of the dam walls and encapsulates each chip in isolation from the surrounding chips. The entire assembly is then baked to harden the epoxy walls and cut along the lines of the walls between adjacent sensors to produce a plurality of individual, encapsulated sensor devices.
This technique is very cost effective, but has a significant disadvantage. Resin can bleed out of the dam wall material when in the liquid state and run onto the chip surface. In some instances, the epoxy may bleed onto the critical image sensing array area causing unacceptable image blemishes. Accordingly, the manufacturing yield is reduced and the unit cost of the sensor device is increased. This problem applies to both monochrome and color sensors, the latter having a thin layer (typically less than 2 microns) of color filter material (the “mosaic”) covering the sensitive array area.